The growing popularity of portable electronic devices such as digital cameras and notebook computers has increased demand for inexpensive, high capacity, high performance non-volatile memory.
A conventional example of a non-volatile semiconductor memory suitable for use in portable devices is flash memory. Flash memory is described in U.S. Pat. No. 4,203,158 to Frohman-Bentchkowsky et al. Flash memory utilizes an insulated floating gate to trap and retain charge over long periods of time. The states of flash memory storage elements represent binary states. Conventional flash memory devices, such as the Frohman-Bentchkowsky device, are fabricated on silicon wafers in batch processes. Silicon wafers are expensive, raising the cost of the flash memory devices, and batch processes result in low production rates.
One approach to cost reduction in memory devices is the use of transistor devices fabricated on plastic substrates. Plastic substrates are generally less expensive than crystal silicon wafers. However, low temperature oxides are used to form the floating gate insulator in these devices. Low temperature oxides are used because of the low melt temperature of the plastic substrate. The volatility of low temperature oxides negatively affects long-term charge storage capability.
One approach to the volatility problem is presented in U.S. Pat. No. 5,360,981 to Owen et al. Owen utilizes structural changes in memory cells to represent digital data. Owen's memory cells, however, are two-terminal devices. Two-terminal devices require a read current to pass through a selected memory cell in order to read the selected cell. This is undesirable because the read current may inadvertently alter the state of the selected memory cell. Therefore, two-terminal memory cells may not be robust enough for some applications.
A memory array requires additional circuitry to read and to write to the memory array. The fabrication of the circuitry is preferably compatible with memory array fabrication. In two-terminal memory devices, diodes are integrated into the memory cell to simplify the memory system design, at the expense of more complicated manufacturing processes. Diodes are also integrated into the peripheral addressing circuits to provide manufacturing compatibility with the memory cells. Diode addressing, however, requires high system power in order to achieve high speed operation.
A need therefore exists for an inexpensive memory array having robust memory cells. A need also exists for a memory device capable of high speed operation without consuming excessive power.